Exit desktop windowing transition

- Added `FreeformTaskTransitionStarterInitializer` to extract transition
  starter setup logic from `FreeformTaskTransitionHandler`.
- Added `DesktopMixedTransitionHandler` to coordinate desktop task close
  transition animation between Launcher (via `dispatchTransition`),
  `CloseDesktopTaskTransitionHandler` and fallback `FreeformTaskTransitionHandler`.
- Added `CloseDesktopTaskTransitionHandler` to animate close desktop
  window transition, when not leaving desktop mode.

Bug: 331165070
Test: atest WMShellUnitTests:DesktopMixedTransitionHandlerTest WMShellUnitTests:CloseDesktopTaskTransitionHandlerTest
Flag: com.android.window.flags.enable_desktop_windowing_exit_transitions
Change-Id: Ic5bf3673a5f5e8ae2fa22a9d3dbe77cefd244cb9